The invention relates to abrasive tools, particularly vitrified abrasive wheels made with a low temperature bond and containing sintered sol gel alumina abrasive grain. The abrasive tools are characterized by improved resistance to grain degradation from bond firing during manufacture of the tools. The invention further includes a vitrified bond composition suitable for firing at relatively low temperatures such as 600.degree.-875.degree. C.
Abrasive tools comprising seeded or unseeded sintered sol gel alumina abrasive grain, also referred to microcrystalline alpha-alumina (MCA) abrasive grain, are known to provide superior grinding performance on a variety of materials. The manufacture and characteristics of these MCA grains and the performance of these MCA grains in various applications are described in, for example, U.S. Pat. Nos. 4,623,364, 4,314,827, 4,744,802, 4,898,597 and 4,543,107, the contents of which are hereby incorporated by reference.
Vitreous or glass bonded abrasive tools containing MCA grain are commercially useful for grinding precision metal parts and other industrial components requiring consistent and improved grinding performance. To produce these types of abrasive tools with consistent quality, reactions between glass bond components and the MCA grain at typical temperatures encountered during firing of the bond, e.g., 1100.degree.-1400.degree. C. must be avoided. Controlling these reactions minimizes damage to the critical microcrystalline structure of the grain.
To reduce the amount of reaction between MCA grain and vitrified bond, U.S. Pat. No. 4,543,107 discloses a bond composition suitable for firing at a temperature as low as about 900.degree. C. In an alternate approach, U.S. Pat. No. 4,898,597 discloses a bond composition comprising at least 40% fritted materials suitable for firing at a temperature as low as about 900.degree. C. In certain grinding applications these low temperature bonds have demonstrated insufficient mechanical strength to meet commercial objectives prompting development of stronger bonds.
Vitrified bonds characterized by improved mechanical strength have been disclosed for use with either alumina oxide or MCA (also referred to as sol gel alpha-alumina) abrasive grits in manufacturing grinding wheels having improved form holding properties. Such bonds are described in U.S. Pat. No. 5,203,886, U.S. Pat. No. 5,401,284 and U.S. Pat. No. 5,536,283, which are hereby incorporated by reference. These vitrified bonds may be fired at relatively low temperatures (e.g., about 900.degree.-1100.degree. C.) to avoid reaction with high performance, sintered sol gel alpha-alumina abrasive grain. The wheels made with these bonds and MCA grain have shown excellent performance in finishing precision moving parts, particularly ferrous metal parts.
It has now been discovered that by selecting appropriate material components, suitable bonds may be made and fired at about 600.degree.-875.degree., preferably 700.degree.-800.degree. C. In particular, by selecting appropriate contents of boron oxide, alkali oxides and alkaline earth oxides, and by maintaining the correct ratios of boron oxide to alkali oxides, sodium oxide to lithium oxide, and silicon oxide to the combination of alkali oxides and alkaline earth oxides in these bonds, and then formulating a wheel comprising this new bond and MCA grain, abrasive tools having improved G-ratio and grinding performance with reduced power draw and acceptable workpiece surface finish can be produced. These tools are an improvement over the low temperature fired, vitrified bonded MCA grain tools previously known in the art.